Some transmission type image display apparatuses such as liquid crystal display apparatuses and sign boards use a direct surface light source device as a backlight. In recent years, direct surface light source devices having a plurality of light emitting elements as the light source have been used (see, for example, PTL 1).
The direct light source apparatus (surface light source device) disclosed in PTL 1 includes a light source substrate, a plurality of light sources (light emitting elements) that emit blue light disposed on the light source substrate, and a wavelength conversion sheet that is disposed for the light sources with an air layer therebetween, and contains a wavelength conversion material such as a phosphor and a quantum dot. In the surface light source device disclosed in PTL 1, when blue light emitted from the light source enters the wavelength conversion sheet, a part of the blue light is converted into red light and green light by the wavelength conversion material. The blue light, red light and green light are mixed into white light, and emitted out of the wavelength conversion sheet.